Add go1.23 iterator support

[freeformz]

Alternative to #140. provides a method on the Set[T], named Elements that returns an iter.Seq[T].

Also adds support for constructing a set from iter.Seq or iter.Seq2 (keys or values).

For go1.22 and below provide the same function with docs but panics if the function is called (noted in the docs).

For go1.23+ add deprecation notice to the old iterator functions.

I used the locally preferred name of Elements based on the convo in #140 & #141, but based on:

• https://pkg.go.dev/slices#Values && https://pkg.go.dev/maps#Values adopting Values
• https://pkg.go.dev/iter#hdr-Naming_Conventions adopting All instead.

Let me know if I should make a change.

Also, let me know if you want me to move the method off the type to a package function like in #140 .

Also updated the test matrix to support the latest go 1.21, 1.22, 1.23, and 1.24 releases.

This was done this way to provide the most seamless path I could think of atm and still be a method of the type.
Once support for version of go < 1.23 are dropped the additional files can be dropped and the contents integrated back into their respective locations.

As an alternative maybe do a V3 release that:

1. supports only go1.23+
2. Drops the old Iter and Iterator methods.
3. Any other API cleanups you want to do.